PSA provides an efficient and economical means for separating a multi-component gas stream containing at least two gases having different adsorption characteristics. The more strongly adsorbable gas can be an impurity which is removed from the less strongly adsorbable gas which is taken off as product or the more strongly adsorbable gas can be the desired product which is separated from the less strongly adsorbable gas. For example, it may be desired to remove carbon monoxide and light hydrocarbons from a hydrogen-containing feed stream to produce a purified (99+ percent) hydrogen stream for a hydrocracking or other catalytic process where these impurities could adversely affect the catalyst or the reaction. On the other hand, it may be desired to recover more strongly adsorbable gases, such as ethylene, from a feedstream to produce an ethylene-rich product.
In PSA, a multi-component gas is typically fed to at least one of a plurality of adsorption zones at an elevated pressure effective to adsorb at least one component, while at least one other component passes through. At a defined time, the feedstream to the adsorber is terminated and the adsorption zone is depressurized by one or more countercurrent depressurization steps wherein pressure is reduced to a defined level which permits the separated, less strongly adsorbed component or components remaining in the adsorption zone to be drawn off without significant concentration of the more strongly adsorbed components. Then, the adsorption zone is depressurized by a countercurrent depressurization step wherein the pressure on the adsorption zone is further reduced by withdrawing desorbed gas countercurrently to the direction of the feedstream. Finally, the adsorption zone is purged with the effluent from an adsorption bed undergoing a countercurrent depressurization step and repressurized. The final stage of repressurization is typically with product gas and is often referred to as product repressurization. In multi-zone systems, there are typically additional steps and those noted above may be done in stages. U.S. Pat. No. 3,176,444 (Kiyonaga), U.S. Pat. No. 3,986,849 (Fuderer et al.) and U.S. Pat. Nos. 3,430,418 and 3,703,068 (both issued to Wagner), among others, describe multi-zone, adiabatic PSA systems employing both countercurrent and countercurrent depressurization and the disclosures of these patents are incorporated by reference in their entireties. The above-mentioned patents to Fuderer et al. and Wagner are herein incorporated by reference.
Various classes of adsorbents are known to be suitable for use in PSA systems, the selection of which is dependent upon the feedstream components and other factors generally known to those skilled in the art. In general, suitable adsorbents include molecular sieves, silica gel, activated carbon, and activated alumina. For some separations, specialized adsorbents can be advantageous. PSA generally employs weak adsorbents and is used for separations wherein the amount of the component to be separated can range from traces to greater than 95 mole percent. PSA systems are preferred when high concentrations of valuable feedstock, products, or reusable solvents are to be recovered. A PSA cycle is one in which the desorption takes place at a pressure much lower than adsorption. In some applications, the desorption takes place under vacuum conditions--vacuum swing adsorption (VSA). To overcome the inherent low operating loadings on the weak adsorbent, PSA cycles generally have cycle times that are short--on the order of seconds to minutes--to maintain reasonably sized adsorbent beds.
One of the problems of building modern gas processing facilities is that the size of the facility or the amount of gas to be treated in any one facility is continuing to increase. Capacities of modern gas processing complexes are generally greater than about 110 thousand normal cubic meters per hour (100 million standard cubic feet per day). Most PSA vessels are limited to a diameter which can be transported to a construction site which generally limits the vessels to a diameter of about 4 meters (about 13 feet) and the height of the vessel is limited by the crush strength of the adsorbent particle. For capacities greater than about 110 thousand normal cubic meters per hour (100 million standard cubic feet per day), PSA processes are provided in multiple trains of duplicate equipment such as pumps, heaters, lines, valves, vessels, and compressors.
It is an objective of the present invention to provide a PSA process for very large gas processing units in a single train of equipment.
It is an objective of the present invention to provide a process sequence which overcomes the physical limitations of vessel size and adsorbent strength to permit the processing of large amounts of feed without giving up overall performance of large-scale gas separation systems.